YA Pair: Female Assassins.
Two of them already this year:
In Grave Mercy, Ismae Rienne puts her natural immunity to poison, her super-fast healing abilities, and her Internal Impending Death Radar to good use in serving Mortain, the god of Death. Intrigue, romance, mystery, and excitement: this one's got it all. And on top of all of that, it's fabulously well-written and plays with genre conventions and is just all-around awesome. Easily one of my favorites of the year.
In Throne of Glass, meanwhile, eighteen-year-old assassin Celaena Sardothien is coerced into competing in a winner-takes-all contest between warriors and criminals. It's great fun, though your suspension-of-disbelief muscle will get a serious workout, and you'll need to ask your cliche-meter to take a nap while you read.
I didn't add Graceling, because I don't really think of Katsa as an assassin. What do you think? Should I? And are there others?